A drive shaft, which transmits power from an engine of an automobile to a drive wheel, needs to cope with angular displacement and axial displacement that are accompanied with a change in relative positional relationship between the engine and the wheel. Accordingly, the drive shaft generally has the following structure. Specifically, a plunging type constant velocity universal joint is provided on the engine side (inboard side), and a fixed type constant velocity universal joint is provided on the drive wheel side (outboard side). Both the constant velocity universal joints are coupled together by a metal intermediate shaft.
The plunging type constant velocity universal joint and the fixed type constant velocity universal joint, which are incorporated in the drive shaft, each include a metal outer joint member including: a cup section for receiving internal parts including an inner joint member coupled to the above-mentioned intermediate shaft; and a stem section extending from the cup section integrally in an axial direction.
Automobile parts including the outer joint member of the constant velocity universal joint situated on the inboard side, the outer joint member of the constant velocity universal joint situated on the outboard side, and the intermediate shaft for coupling both the constant velocity universal joints together are generally hardened by induction quenching as heat treatment in order to increase strength and the like of the automobile parts. After the induction quenching, the automobile parts are tempered in order to increase toughness of the automobile parts, to partially release stress accompanied with the quenching, and to forestall a quenching crack. Further, in order to improve corrosion resistance, a coating agent is applied onto an outer surface of each of the automobile parts, and is baked by heating. In this manner, the automobile parts are subjected to rustproofing.
As tempering of each automobile part described above, a method of using a hot-air furnace and a method of using induction heating are adopted. In the method of using a hot-air furnace, the automobile part is heated for about 40 to 60 minutes in the furnace at a temperature of about 160 to 185 C.°, and the automobile part is air-cooled after completion of heating. Thus, tempering is completed. At the time of the tempering, the coating agent is previously applied onto the outer surface of the automobile part that has been quenched, and the coating agent is sometimes baked using heating for the tempering.
On the other hand, in tempering using induction heating, the automobile part is often heated for about 15 to 30 seconds, and a heating temperature needs to be increased to about 220 to 270 C.°. Accordingly, a heating time is shorter and the heating temperature is higher than those of the method of using a hot-air furnace, and hence the tempering using induction heating is not suitable for baking the coating agent. In this case, in actual use, a coating step is additionally provided, and the coating agent is baked using a hot-air furnace. Further, a method of baking the coating agent using induction heating has been proposed. However, in this case, the method is designed only to bake the coating agent, and hence does not satisfy a treatment condition that baking and tempering are compatible (for example, see Patent Literatures 1 and 2).